1. Field of the Invention
The present invention relates generally to laser detection and ranging system which are commonly referred to as LADAR systems. More specifically, the present invention relates to a method, implemented with a computer software program, for improving direct detection solid state laser (LADAR) imaging.
2. Description of the Prior Art
In the past, methods and apparatus have been developed for identifying targets, whether such targets have been stationary or moving. For example, in World War II the British developed and utilized radar for identifying the incoming aircraft of the German Luftwaffe. Radar uses radio waves (instead of the light waves that human eye uses to see) to locate objects at great distances even in bad weather or in total darkness. Currently, radar is employed in a wide variety of areas, including air traffic control, early warning systems for national defense, law enforcement of speed limits on highways, and weather forecasting, along with the more traditional use of radars which is target identification.
While ground-based and airborne radar are used to identify enemy ships, land based vehicles such as tanks and aircraft, radar is also used to aim and fire guns and missiles. In addition, radar has also been used to map terrain. While radar has proven quite effective in many areas over the years, radar is inherently limited because of its use of radio frequency signals and the size of the resultant antennas used to transmit and receive such radio frequency signals.
In an effort to overcome some of the limitations of radar systems, lasers have been employed in detection and ranging systems. Like radar, which takes its name from the phrase "radio detection and ranging," laser detection and ranging systems are referred to as LADAR systems. Like radar systems, which transmit radio waves and receive waves reflected from objects, often referred to as targets, LADAR systems transmit laser beams and receive the beams reflections from targets.
Because of the short wavelengths associated with laser beam transmissions, LADAR imaging exhibits a much greater resolution than radar imaging. In other words, LADAR is capable of accurately pinpointing a target when compared with the ability of radar to pinpoint a target.
The LADAR systems currently in use form two different types of images. However, before describing these images, it should be understood how these images are formed. A scan pattern is generated by scanning a pulsed laser beam in a pattern throughout a particular field. The generation of this scan pattern is somewhat similar to the manner in which an electron gun in a television set is rastered many times a second over the entire screen to generate a television image. In fact, LADAR images are frames of pixels having x rows and y columns to form a pixel array. The pixel array may be any of a variety of sizes, a common array size is 256 pixels per row and 256 pixels per column. One of the images generated in this manner is referred to as the "range" image. The range image is generated by determining the time between the departure of a laser pulse and the corresponding return laser pulse that has been reflected from an object in the pattern field. For display purposes, each range image is divided into range bands. A repeating color spectrum is assigned to each range band in the range image so that relative distances of objects in the range image can be perceived. For example, objects in a particular range image may be from 1,000 to 2,000 meters away from the LADAR, and the color bands may each be 100 meters. In this example, the range image would show ten repeating color bands. Each color band might use, for instance, five different colors to illustrate the relative range of an object within the color band. Blue might be used to represent the range of each object in the first 20 meters of a color band, with the colors green, yellow, red, and purple used to represent the range of objects in successive 20 meter bands of each color band.
The other type of image generated by a LADAR system is referred to as an "intensity" image. The intensity image is generated by determining the magnitude of each reflected laser pulse received by the LADAR system. Highly reflective objects in the field, such as light colored objects, appear lighter in the intensity image because the laser pulse reflected from these materials will have a relatively high magnitude. Poorly reflective materials, such as dark colored objects, appear darker in the intensity image because these materials absorb much of the transmitted laser pulse and, thus, the reflected pulse received by the LADAR system is relatively weak.
While the range and intensity images are useful for a variety of purposes, the intensity image has at least two unique uses. First, it provides an image that is typically easier for a person to interpret than the range image, because the intensity image looks very much like a black and white picture. The range and intensity images are typically viewed together, so that a person can identify features in the intensity image and then look to the range image to gain some understanding of the three dimensional aspects of the identified features. Second, the intensity image can be processed using automatic target recognition systems that use edge detection to identify and locate fixed, high value targets, such as buildings. For instance, a LADAR system can be mounted into the nose of a missile, and the data provided by the LADAR system can be used to guide the missile to the target. If the mission target is a fixed, high value target, an automatic target recognition system might be used to detect the target by detecting edges of the target appearing in the intensity image provided by the LADAR system.
For direct detection LADAR systems such as the Lockheed Martin Corporation Vought solid state LADAR, the amplitude or intensity of the return pulse from a target is a measure of the quality of the signal. A high intensity image corresponds to a high degree of confidence in the accuracy of a range measurement to a target. On the other hand a low intensity image corresponds to a weak signal and the very good probability of noise contamination.
Thus, there is a need for a means to generate sharper intensity images to facilitate and improve the operation of direct detection LADAR systems since it would be easier for such systems to locate targets if such targets were better defined by a sharper intensity image. Of course, sharper intensity images would also enhance the use of LADAR systems in other areas, such as terrain mapping.